Fix render-blocking widget script

[filip1]

The widget embed code prevents the embedding page from rendering while the widget script is loading. Instead, the script can be loaded asyncronously while showing a loading animation.

An example of an asynchronous embed code that loads widget 9123 from policat.org:

<div id="policat_9123"><div class="policat-loading"></div></div>
<script type="text/javascript">window.policat_target_id_9123 = "policat_9123";</script>
<script type="text/javascript" src="https://www.policat.org/api/js/widget/9123" async="true"></script>
<style>
.policat-loading {
    position: relative;
    height: 1em;
    text-align: center;
}
@keyframes policatspin {
  to {transform: rotate(360deg);}
}
.policat-loading::after {
    content: '';
    box-sizing: border-box;
    position: absolute;
    top: .33em;
    width: 1em;
    height: 1em;
    margin-left: 1em;
    border-radius: 50%;
    border-top: 2px solid #333;
    border-right: 2px solid transparent;
    animation: policatspin .8s linear infinite;
}
</style>